1. Field of the Invention
The present invention relates to latent electrostatic image bearing member (hereafter may be referred to as “photoconductor” or “electrophotographic photoconductor”) that can provide high-quality images for prolonged periods, owing to photosensitive layers and crosslinked surface layers having excellent flaw and wear resistance and appropriate electric properties; process cartridge, image forming process and image forming apparatus that utilize latent electrostatic image bearing member respectively.
2. Description of the Related Art
Recently, organic photoconductors (OPC) are widely employed in copiers, facsimiles, laser printers, and composite apparatuses thereof owing to excellent performance and various advantages, in place of conventional inorganic photoconductors. The reasons for replacement are, for example: (1) favorable optical properties such as absorbable wavelength region and absorption rate, (2) electrical properties such as high sensitivity and stable charging ability, (3) broad selection of materials, (4) manufacturability, (5) low cost, (6) no toxic effects, etc.
On the other hand, photoconductors are more and more miniaturized as image forming apparatuses are being downsized; in addition, the trend toward speeding up and maintenance-free performance of machines are spurring the demand for ruggedization of photoconductors nowadays.
However, organic photoconductors, due to their relatively low hardness of surface layers that consist mainly of low-molecular charge transport substances and inactive polymers, tends to wear away under repeated usages in electrophotographic processes by the mechanical stress associated with developing systems or cleaning systems, etc.
To pursue high image quality, rubber hardness of cleaning blades as well as the pressure applied onto the photoconductors is being forced to increase so as to improve cleaning ability accompanied by the miniaturization of toner particles, therefore accelerating the wear on photoconductors. This kind of wear on photoconductors deteriorate sensitivity and electric properties such as charging ability etc., resulting in disordered images such as image density degradation or background smear, etc. Flaws caused by local wears often bring about streaks on images due to insufficient cleaning. Such wear and flaws typically dominate the cause of short lives of photoconductors that are being exchanged shortly.
Therefore, it is essential to reduce the amount of wear for improved durability of organic photoconductors, and it is the most significant problem in the field to be settled in a prompt manner.
Technologies to improve wear resistance of photosensitive layers, for example, (1) incorporation of curable binders into the crosslinked charge transporting layers e.g. Japanese Patent Application Laid-Open (JP-A) No. 56-48637, (2) employment of charge transport polymers e.g. JP-A No. 64-1728, (3) dispersion of inorganic fillers into crosslinked charge transporting layers e.g. JP-A No. 4-281461, and the like are proposed.
However, the technology incorporating curable binders described in (1) has insufficient compatibility with charge transport substances and residual voltage tends to increase owing to impurities such as polymerization initiators and/or unreacted residual groups, it is more likely to deteriorate image density. The technology employing charge transport polymers described in (2) can improve wear resistance in some measure; however, durability of organic photoconductors does not improve sufficiently. Moreover, electric properties of organic photoconductors are likely to become unstable because of difficulties in polymerizing and purifying charge transport polymers. Furthermore, coating liquids typically become excessively viscous for processing.
The technology in which inorganic fillers are dispersed as described in (3) may exhibit higher wear resistance compared to the conventional photoconductors in which the low-molecular charge transport substances are being dispersed into inactive polymers, however, charge traps on the surfaces of inorganic fillers tend to increase residual potential, thereby increasing the tendency for image density degradation. Also, if unevenness of inorganic fillers and binder resin of photoconductor surface is significant, defective cleaning may occur, resulting in toner filming or image deletion.
Based on these technologies (1), (2), and (3), the overall durability of organic photoconductors including electrical and mechanical durability has not achieved the satisfactory level.
Photoconductors containing cured materials of multi-functional acrylate monomers are proposed in order to improve wear and flaw resistance described in (1) e.g. JP-B No. 3262488. It is described as cured materials of multi-functional acrylate monomers are to be contained in the protective layer disposed on photosensitive layers, however, there is no specific description or examples other than the charge transport substances may be contained in the protective layers. Furthermore, when low-molecular charge transport substances are added to the crosslinked charge transporting layers, compatibility issue may arise with the cured materials. As a result, deposition and clouding of low-molecular charge transport substances may occur, in addition to the image density deterioration and reduced mechanical strength due to the increase in exposed-area potential.
More specifically, photoconductors are produced by reacting monomers with polymer binders being incorporated; therefore, three dimensional networks do not proceed sufficiently and the crosslinked joint density becomes less, failing to achieve a dramatic increase in wear resistance.
To improve wear resistance of photosensitive layers, for example, disposing charge transporting layers produced by the use of coating liquids with monomers having carbon-carbon double bonds, charge transport substances having carbon-carbon double bonds, and binder resin is proposed in JP-B No. 3194392. The binder resin is thought to improve adhesiveness between charge generating layers and curing charge transporting layers and alleviate the internal stress of film at the time of thick film curing. The binder resin can be classified broadly into two categories: binders reactive to the charge transport substances having carbon-carbon double bonds, and binders non-reactive to the charge transport substances having no double bonds. This photoconductor is remarkable in having wear resistance and proper electrical properties, however, if non-reactive resins are used as binder resin, compatibility with cured materials generated from reactions with monomers and charge transport substances may not be desirable and a phase separation within crosslinked charge transporting layers may occur, resulting in flaws or retention of external additives of toner and paper powders. As stated above, three-dimensional network does not progress appropriately and the crosslinked density becomes sparse, prohibiting the exhibition of significant wear resistance. In addition, monomers specified in Japanese Patent (JP-B) No. 3194392 have two functionalities, not sufficient for wear resistance. When a reactive resin is employed as binder resin, though the molecular mass of cured materials increases, number of intermolecular crosslinked joints is small, thus simultaneous pursuit of bonding amount and crosslinked density of the charge transport substances is difficult and electric properties and wear resistance would not be satisfactory.
Photoconductors having photosensitive layers that contain cured compounds generated from curing hole transport compounds having two or more chain polymerizable functional groups within one molecule is proposed in Japanese Patent Application Laid-Open (JP-A) No. 2000-66425. The photosensitive layer may have high degree of hardness due to increased crosslinked joint density, however, since bulky hole transport compounds have two or more chain polymerizable functional groups, distortion within cured materials may occur and internal stress becomes high and the crosslinked surface layers may yield cracks or peelings when used on long-term basis.
From these aspects and much dedicated investigations on the subject, it is found that employing crosslinked resin layer obtained from curing radical polymerizable compounds having three or more functionalities with no charge transport structure and radical polymerizable compounds having one functionality with charge transport structure as surface layer improves electric properties and wear resistance. However, this crosslinked resin layer is electrically unstable; specifically, charge deterioration has been verified after long-term use. These are assumed to be caused by the decomposition or alteration of charge transport substances or binder resin led by the eruption of NOx or ozone gases from outside or within the electrophotographic apparatus. Specifically, it is thought to be caused by the deteriorated outer surface where deterioration is most likely to be progressed, that has been retained for a long period of time because of improved wear resistance due to disposed surface protective layers.
Examples of effective countermeasures to above issues include employing photoconductors on which protective layers containing fillers, dispersants and at least two different antioxidants are disposed, as disclosed in JP-A No. 2002-207308, or employing photoconductors containing protective layers with charge transport property, syloxane resin with crosslinked structure and antioxidant, as disclosed in JP-A No. 2001-51440. However, when small amount of hindered phenol antioxidant or hindered amine antioxidant as described in above literatures are contained in the surface layers made of crosslinked resin layers produced from the curing radical polymerizable compounds having three or more functionalities with no charge transport structure and radical polymerizable compounds having one functionality with charge transport structure, the protective layers show high wear resistance compared to the conventional protective layers and the outermost surface will not get refreshed and insufficient electric stability, especially the charge property deterioration will result in long-term use. If antioxidant is contained in excessive amount, the wear resistance is deteriorated due to sensitivity degradation or crosslinking inhibition.
On the other hand, in terms of image-forming apparatuses, dehumidification of photoconductors is known as a way to refresh outermost surfaces of photoconductors, however, employing heaters will not only incur size-growth of apparatus but consumes large volume of electricity, therefore is not cost-effective. To overcome this problem, for example, the image-forming apparatus in which an exhaust path and a fan is placed between photoconductor and fixing unit so that the waste heat from the fixing unit is sent to the photoconductor via exhaust path or duct as disclosed in JP-A No. 08-179677. However, the fixing unit has a temperature as high as around 200° C. when it is under operation and the surface of photoconductor becomes hot in the image forming apparatus where the heat of fixing unit is directly transmitted to the photoconductor and the sensitivity of photoconductor is deteriorated.
Therefore, the image forming processes and associated technologies with superior endurance that can provide high-quality images for prolonged periods, having latent electrostatic image bearing members that can provide high-quality images for prolonged periods, owing to photosensitive layers and crosslinked surface layers having excellent flaw and wear resistance and appropriate electric properties, have not been obtained and in the present state of affairs, their prompt development is desirable.